Polyolefins are widely used in various industries such as food, pharmaceutical, agriculture, automobile which include preparation of moulding products such as kitchen ware, food containers, drink bottles, garden furniture, washing machine components, automotive parts, ropes, tapes, twines, fishing nets, flexible containers etc.
The use of polyolefins for the said applications requires improvement in impact strength, stiffness, heat resistance and fast injection moulding cycle times which are all achievable due to higher crystallization temperatures obtained when using high quality nucleating agents.
This improvement in the physical qualities of polyolefins can be achieved by using nucleating agents that give enhanced physical and clarifying properties to polymer.
Nucleating agents are generally inorganic materials with a small average particle size and a high melting point.
The use of nucleating agents is economically beneficial, as they increase the polymers' crystallization temperature, which leads to shorter processing cycles and associated energy savings. A sub-group of nucleating agents consists of ‘clarifying’ agents, i.e. additives that are used to improve the optical properties of a polymer.
Polypropylene is recognized as a relatively easy material to nucleate, where the rate of crystallization is low enough to allow the nucleating agent to have a direct impact on the nucleation density. Conversely, polyethylene has an extremely fast rate of crystallization, rendering most nucleating agents as ineffective. Further Isotactic polypropylene (iPP) is widely used polymer, due to its good mechanical properties and low price. However, iPP requires very high undercooling for crystallization to take place, due to its bulkier chains and more complex crystal unit cell arrangement compared to, polyethylene (PE). Thus, nucleating agents can offer a great advantage in industrial processing times, as well as significantly influence the solid-state structure of iPP.
Typical nucleating agents known in the art include minerals, such as chalk, clay, kaolin, talc, silicates, and the like. Organic nucleating agents, such as salts of aliphatic or aromatic carboxylic acids, aromatic salts, metallic salts of aromatic phosphorus compounds, quinaridones, aromatic amides, and polymers having melting points are more efficient.
A range of potential nucleating agents including organic acid derivatives, sorbitol derivatives, phosphorus-containing species, and inorganics are known in the art. Few representative examples of commercially available nucleating agents are described herein below.
Jansen et al, in Plastics Additives Handbook, by Gachter & Muller, pp. 862-875,1993, discloses nucleating agents and their effect on crystalline and semi-crystalline polymers.
U.S. Pat. No. 6,537,665 discloses certain alumina-coated silica nano-powders carrying a surface coating of an organic acid also function as polymer nucleating agents.
H. N. Beck in Journal of applied polymer science 11, pp. 673-685 (1967) discloses heterogeneous nucleating agents for polypropylene crystallization such as sodium benzoate and basic aluminum dibenzoate.
U.S. Pat. No. 6,913,829 describe nucleating agent, particularly alumina-coated silica particles bonded to benzoic acid, optionally in presence of surfactant.
Zhiping Lv et al. in Materials and Design 37 (2012) 73-78 reports design and properties of a titanium based nucleating agent for isotactic polypropylene (iPP) selected from group consisting of titanium ester, particularly TB, TD and TSD (titanate of benzoate or 4-tert-Butylbenzoate) and process for preparation thereof, wherein, silicon titanium esteri.e. (TSD) was found to be effective nucleating agent for crystallization of iPP.
U.S. Pat. No. 7,144,939 relates to the use of organic nucleating agents such as cyclic dicarboxylates, including metal salts of hexahydrophthalic acid (HHPA) to induce certain specific polymer crystalline orientations into the molded polypropylene articles and improved physical properties as well.
The effect of the calcium salts of suberic (Ca-Sub) and pimelic (Ca-Pim) acids additives on the crystallization, melting characteristics, and structure of isotactic polypropylene (iPP) was studied by József Vargaand reported in Journal of Applied Polymer Science 74. (10), pp 2357-2368, 1999.
Markus Blomenhofer et al. reported organic compounds, particularly substituted 1,3,5-benzenetrisamides, capable of selectively and extremely efficiently nucleating the growth of the common α-crystal modification of isotactic polypropylene (i-PP), in Macromolecules, 2005, 38 (9), pp 3688-3695
Further U.S. Pat. No. 4,536,531 describes use of carboxylic salts of alkali or alkali earth metals as nucleating agents for polyesters, wherein metal salts of aliphatic monocarboxylic acids such as acetic acid, propionic acid, caproic acid, palmitic acid, stearic acid, oleic acid, behenic acid, montanic acid etc. and suitable metals are sodium, potassium, lithium, magnesium, calcium, barium, and zinc.
JP 48074550 describes use of metal salts of aryl phosphonates such as calcium benzylphosphonate and aluminium phenyl phosphonate as nucleating agents for polyesters such as polybutylene terephthalate (PBT), wherein use of 0.5 wt % zinc phenylphosphonate in PBT improves the properties compared with PBT alone.
In view of above, many of the nucleants have shown effectiveness in increasing the nucleation density of thermoplastic polyesters such as PBT, PHA, whereas nucleation of semi-crystalline polymers requires aliphatic carboxylic acid, phosphoric acid etc.
Though the above nucleants or nucleating agents provide the sites or nuclei for initiating polymer crystallization, certain disadvantages are accompanied with it, such as agglomeration which can generate regions of stress concentration and in homogeneity in moulding, opaque products, environmentally undesirable and toxic; additionally some of the nucleating agents are commercially expensive and leads to poor crystallization. Thus, there is a need for benign and cost-effective class of nucleant which allow for the production of semi-crystalline thermoplastic polymer like polyolefins having high crystallinity, strength, dimensional stability.
U.S. Pat. No. 6,369,183 describes the preparation of polymer composite and hybrid polymers and resins in which at least one component is a chemically modified carboxylate-alumoxane. The carboxylate alumoxanes are chemically bonded into the polymer backbone through reaction of appropriate functional groups of the polymer precursor with carboxylate alumoxane at higher proportion (more than 5 wt %). Notably, it has not been used as nucleating agents. Where as in the present invention a tiny amount (3 ppm) of carboxylate-alumoxane is added to nucleate Polypropylene (PP) in order to speed up the crystallization of PP.